As you might expect, the inside length is the driving dimension of these cars, and it isn't always tied to the overall length of the car-- i.e. there is no standard bulkhead "thickness" or placement. For example, WM rebuilt both 50' and 53'6" flats with bulkheads and steel V-decks for pulpwood service, with the same bulkheads and same inside length on both versions. The bulkheads were inset on the longer cars. This way the two could be used interchangeably.
What's really interesting is that WM, probably following C&O's lead, permanently coupled pairs of those pulpwood cars with drawbars! All were eventually separated aside from some odd Greenville cars which seem to have been scrapped in that configuration.